Progressing cavity pumps (PCP) were invented in the 1930s by Moineau as seen in U.S. Pat. Nos. 1,892,217 and 2,028,407.
A progressing cavity pump has a stator and a rotor. The stator typically comprises an elastomeric liner within a housing. The stator is open at both ends and has a multi-lobe helical passage extending through it. The rotor is normally of metal and has a helical exterior formed on it. Rotating the rotor causes fluid to pump through the stator. Progressing cavity pumps are used for a variety of purposes.
As a well pump, progressing cavity pumps may be driven by a downhole electrical motor or by a string of rods extending to a motor located at the surface. With a rod driven pump, normally the stator is suspended on a string of tubing, and the drive rods are located within the tubing. When installing a rod driven progressing cavity pump, the operator first secures the stator to the string of tubing and runs the tubing into the well to a desired depth. The operator then lowers the rotor through the tubing on the string of rods and into the stator.
To operate the pump at desired capacity, the rotor must be at the desired axial spacing within the stator and the rods must be in tension. If the lower end of the rotor is spaced above a lower end of the stator during operation, then a lower portion of the stator will not be in engagement with the rotor and the pumping capacity will suffer. The operator thus needs to know when the rotor has fully entered the stator during installation. The operator can calculate how much the rods will stretch due to the hydrostatic weight of the column of well fluid in the tubing. With the anticipated stretch distance known and with the rotor at a known initial position in the stator, the operator can pull the rods and rotor upward a distance slightly greater than the anticipated stretch, so that during operation, the rotor will move back downward to the desired axial position relative to the stator.
Stators are manufactured by insertion of a core into a tubular housing and capping the ends with the core properly positioned. The inside wall of the housing can have an adhesive coating before the material for the stator is injected through one of the end caps and forced under pressure to fill the annular space between the core and the housing inner wall. The adhesive was used in the past to help the stator body adhere to the surrounding housing. Depending on the size and the particular application, the housing could be over 10 meters long and could have an inside housing wall diameter smaller than 10 centimeters.
As the industry develops, PCPs are being deployed in progressively hotter environments to the point where the commercially available adhesives reach their temperature service limit in the order of about 150° C. In an effort to allows stators to operate effectively at higher temperatures structures have been proposed to be supported from the housing inside wall and extend inwardly such that when the stator was created within the housing a core and injected rubber around it, the end result would be a better bond to the housing inside wall than just using adhesive by itself. Along those lines U.S. Pat. No. 7,407,372 suggests a ring structure with openings that allow the rubber to pass through during manufacturing and positioned in the stator housing with L-shaped rings 18 that are welded to the stator inside wall as shown in FIGS. 2 and 3 of that patent. FIGS. 4 and 5 show another embodiment of such a ring with openings and external grooves 52 that lead to openings 54 so that the rubber can hopefully envelope the ring structure 50. The grooves are stated to be longitudinal or spiral and FIG. 5 further shows L-shaped indents at opposed ends into the ring 50 from the inside that are stated to help seal the rubber to the ring structure 50.
There are several issues with this design. In a long housing it is expensive and difficult to secure the intermediate standoff supports 18 to the housing inner wall. The more substantial the tube for structural rigidity the less rubber can be used as the stator. On the other hand if the tube is too flimsy so as to maximize the rubber content it will be structurally weaker to the point that during stator manufacturing with the core in the housing and the ring held by supports, the delivery of rubber under very high pressures to fill all the void space between the housing inner wall and the core will result in flexing of the tube to the point where it will touch the core. When the core is then removed portions of the tube extend out of the stator and damage the rotor.
Other references relating to PCP stator construction are: U.S. Pat. Nos. 3,280,753; 5,318,416; 7,131,827; JP 61180512; DE 3322095; US 2009/0152009; 2009/0169404; 2002/0153141; 2009/0129937; U.S. Pat. Nos. 7,299,873; 7,201,222; 6,868,912 and 6,705,402.
What is needed and provided by the present invention is a simple way to enhance grip of the stator to its housing that is structurally sound against torsional stresses and offers in some embodiments the ability to stiffen the stator. This is accomplished with modifications to a tubular housing for the stator that can have elongated ribs extending inwardly from the housing inner wall disposed longitudinally or in a spiral array. The spiral array can have ribs spiraling all in one direction or with one or more ribs spiraling in the opposite direction forming an overlapping pattern of ribs. These ribs are formed as an integral part of the housing either by extrusion, machining, or welding such that they cannot move with respect to the housing during injection of the stator rubber or due to torsional stresses during operation. The reverse of inwardly extending ribs can also be used in the form of wall grooves in the stator housing interior wall that preferably have a bulbous region further into the wall from a narrower inlet so that a grip is created when the internal groove structure is filled with injected rubber to form the stator. These and other aspects of the present invention will become more readily apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is defined by the literal and equivalent scope of the appended claims.